Interference filters rely on constructive and destructive interference to provide and shape filter responses. The interference is created by overlapping different phase-shifted portions of the same beam. The beam divisions are generally overlapping portions separated only by time (i.e., phase delay). However, the beams can also be divided spatially into transverse sections subject to different phase delays, which must be recombined to produce the required overlap.
Examples of interference filters with temporally separated beam portions include dielectric filters, Fabry-Perot etalons, Bragg gratings, long period gratings, and micro-optic devices. Most of these filters, particularly Fabry-Perot etalons and long period gratings, have limited response profiles and must be concatenated to produce more complex response profiles. For example, simple response profiles, such as Gaussian profiles, can be combined by conventional curve fitting techniques to approximate the desired response profiles, however, the multiple filter components can be cumbersome to assemble and are subject to both fabrication and assembly errors.
Co-assigned U.S. Pat. No. 5,841,583 to Bhagavatula entitled "Multi-path Interference Filter" discloses examples of interference filters with spatially separated beam portions. Complex response profiles can be supported by the multi-path filters, but fitting the filters' performances to desired response profiles is more difficult.